Date: Wed, 23 Nov 2005 14:09:09 -0800

Author: Paul Doherty

Subject: Re: Airplane wing calculations

Post:

In atmospheric pressure air, engineers do use neither bernoulli
calculations nor air flow deflection calculations to calculate the lift
of a wing given wing shape, airflow speed, and angle of attack.

As an example of some of the difficulties, most of the air deflected
downward by the wing is deflected downward by air flowing over the top
of the wing, that's why gliders have spoilers on the top of the wing
not on the bottom.

Paul D

On Nov 23, 2005, at 1:37 PM, Chuck Patten wrote:

> Think of the symmetrical airfoils or non-symmetrical ones as any rigid
> flat
> plane suspended in a medium. "Flight" is the movement of the plane
> surface
> within the medium. The velocity and shape of the airfoil or plane
> surface
> together determine the angle of attack needed to control the momental
> direction of the object within a three dimensional space.
>
> cheers,
> chuck...
>
> -----Original Message-----
> From: owner-tap-l@listproc.appstate.edu
> [mailto:owner-tap-l@listproc.appstate.edu] On Behalf Of Thomas J. Bauer
> Sent: Wednesday, November 23, 2005 9:28 AM
> To: tap-l@listproc.appstate.edu
> Subject: Re: Airplane wing calculations
>
> tap-l@listproc.appstate.edu on Wednesday, November 23, 2005 at 9:41 AM
> -0500 wrote:
>> Howdy all,
>>
>> Got a question for the list. If I wanted to calculate the lift force
>> on
>> an airplane wing, how would I do it? I know it basically boils down
>> to
>> F=PA, where P can be determined from the Bernoulli effect. However,
>> in
>> Bernoulli's equation, how do you determine the speed of the air moving
>> over the top of the wing as compared to the air moving beneath the
>> wing?
>> This is my point of confusion.
>>
>> Thanks in advance, and I hope everyone has a Happy Torquey Day :)
>>
>> Cheers,
>>
>> Matt Lowry
>
> This is opening a can of Acturian Slime worms.
> If the Bernoulli effect is so important, why do symmetrical airfoils
> fly?
> My understanding is the more important effect is to imagine a thin flat
> wing, such as a piece of cardboard. Think of the wing interacting with
> the
> air molecules that strike it on the bottom. These molecules elastically
> bounce off the bottom of the wing. Relative to the wing, they have a
> net
> change in momentum of these downward. This creates a force on the wing,
> which is the lift. (there is also a component in the forward direction
> which produces drag.) Unfortunately the the volume of air that the
> cross-sectional area of the wing times the sine of the angle of attack
> does not produce the amount of lift that is generated by an actual
> wing.
> The amount of air that is deflected downwards turns out to equal the
> area
> of a circle whose diameter is the span of the wing. This is true as
> long
> as the air passing over the backside of the wing remains "attached" to
> the
> wing surface. Airfoils then just become a way of keeping this laminar
> flow
> working at high angles of attack. This is a very simple model, but it
> does
> work pretty well, when the Bernoulli effect can not explain things
> because
> there is no path difference between going over the top or going over
> the
> bottom to the wing.
>
> You can demonstrate this by videotaping a small cardboard glider as it
> flys through still air. You measure the velocity and angle of attack.
> From
> this you get the velocity of the air traveling downwards. Times the
> area
> of the wing you get the volume/time. Times the density oif air you get
> the
> force. This doesn't equal the weight unless you replace the area of the
> wing with the area of a circle whose diameter equals the span of the
> wing.
> I have had students do this for projects and the numbers work out quite
> well. If you have ever seen an aiplane fly in a snow storm it should be
> apparent that there is more air being deflected than just the
> cross-sectional area of the wing. There was an article about this in
> the
> "Physics Teacher" about three or four years ago.
>
> Tom Bauer
> Wellesley College
>
From jpdulka@cox.net Wed Nov 23 17:16:20 2005

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